The present invention relates to magnetic resonance imaging, and more particularly, to magnetic resonance imaging over a reduced field of view using local gradient spoiling.
Magnetic resonance imaging (MRI) uses strong, homogeneous static and gradient magnetic fields and carefully timed rf pulses to produce noninvasive images of organs or tissues within a patient""s body. MRI provides significant advantages over x-ray, ultrasound, or other imaging techniques. However, MRI produces various image artifacts under certain imaging conditions such as motion.
Other aliasing artifacts occur whenever the field of view (FOV) of the image becomes smaller than the size of the object being imaged. In such a case, the parts of the object that extend outside the FOV are wrapped back into the image on the opposite side. In the readout direction, aliasing can be avoided without affecting the scan time and resolution by oversampling at a higher frequency during the readout period. In the phase-encode direction, the analogous solution is to increase the number of phase-encode lines per image, increasing the scan time.
For applications in which fast, high resolution scans are desired, aliasing artifacts must be addressed in other ways. An example of this is Cardiac MRI, which requires resolution of a few millimeters (mm) or better and must be performed quickly because of cardiac and respiratory motion. Yet, the region of interest (the heart) is small compared to the subject size, and if the FOV could be reduced without aliasing artifacts compromising the image quality, large savings in scan time would result.
One way to address the reduced FOV aliasing problem is to reduce or eliminate the magnetic resonance (MR) signal from outer regions of the subject, i.e., those regions which would wrap back into the region of interest when the FOV is reduced. Such reduction or minimization of the MR signal is referred to in the art as xe2x80x9cspoilingxe2x80x9d. The ways to accomplish this are broadly classified by whether they use the RF or static fields to achieve spatial differentiation.
In the first category, surface coils are effective in reducing the signal from the interior, but not from the surface. Further, depth pulse sequences using surface coils may effectively suppress the surface signal, but require a long pulse preparation time and rely on a large flip angle, both of which increase the imaging time. Surface saturation may be used, but this method also adds time for the spin preparation and does not entirely eliminate the signal from fast-relaxing regions, e.g., subcutaneous fat. RF shielding with metal-fiber blankets works well in eliminating signals from the extremities, but it is ineffective in screening out the magnetic field from shoulder and torso surfaces when penetration into the torso is to be maintained. A passive eddy current spoiler may eliminate near-surface MR signals, but this technique requires linearly polarized rf, the effective depth is not adjustable in situ, and it may not provide effective spoiling over a large area.
The second approach to reducing the field of view aliasing problem is to dephase the excited spins in outlying regions or to distort their apparent position using local magnetic field gradients. To date, these methods have had only moderate success. For example, ferromagnetic and paramagnetic materials inserted into the scanner establish large, local static field gradients. Unfortunately, the spoil depth for these materials is not adjustable, and large areas cannot simultaneously be spoiled. Higher order shim coils have been used to distort the edges of the image so that it can xe2x80x9choldxe2x80x9d the entire sample or patient even with a small FOV. However, power requirements for this technique are prohibitively large, and a loss of dynamic range results because magnetization from a large part of the patient gets concentrated into a small region of the image.
Two other methods of magnetization spoiling have been used in MR spectroscopy for volume localization, but not for FOV reduction or faster scanning. Topical magnetic resonance (TMR) uses high-order shim coils to make the field homogeneous only in the central region of the magnet. However, incorporating this technique into imaging protocols appears difficult because of the large power requirements in human-sized scanners. In contrast, use of a local gradient insert, such as that designed by Chen and Ackerman for 31P spectroscopy of rat liver has potential in imaging applications as well as in spectroscopy. (W. Chen and J. Ackerman, Spatially-Localized NMR Spectroscopy Employing an Inhomogeneous Surface-Spoiling Magnetic Field Gradient, NMR in Biomedicine, Vol. 3, No. 4 (1990)). The main difficulty with the Chen and Ackerman approach is in scaling up the device to spoil to depths of more than a few millimeters while maintaining modest current requirements.
Accordingly, there exists a need for cost effective methods and related apparatus for the prompt and reliable imaging with a reduced FOV of a large object. The present invention satisfies these needs.
The present invention is embodied in a method, and related apparatus, of spoiling the magnetic resonance signal during imaging to an experimentally adjustable depth by means of a gradient insert, so that the image""s FOV may be reduced without aliasing artifacts and the images may be collected more quickly. The present invention provides a spoiling gradient suitable for human-scale imaging that is capable of achieving controlled-depth spoils of 80 millimeters under a wide variety of scan protocols. The apparatus and method of the invention have been experimentally verified using phantoms and a human subject. Use of the apparatus and method enables imaging times to be reduced by as much as half without loss of resolution.